The present invention relates to apparatus which measure the frequency of an electrical signal; and particularly to such apparatus which detect a predefined reoccurring characteristic of the electrical signal.
The frequency of an electrical signal is measured by detecting a given reoccurring characteristic, such as a maximum, minimum or zero crossing point, in the signal waveform. The amount of time the elapses between each occurrence of such a signal characteristic indicates the period of the signal, which parameter corresponds to the frequency.
For example, a sinusoidal signal has two zero crossings per cycle, i.e. one crossing at the beginning of the cycle and another at the mid point of the cycle; of course there is another zero crossing at the end of the cycle when another signal cycle begins. Detecting one zero crossing per cycle can be employed to determine the frequency or the period of the signal.
Circuits have been developed to detect zero crossings of a signal. However, such prior circuits detected every zero crossing and could not distinguish the zero crossing at the beginning of a cycle from the one at the cycle mid point.